1. Field of the Invention
The present invention relates to an image formation apparatus in which image formation units are arranged in tandem.
2. Description of Related Art
A type of image formation apparatus for obtaining a color image is conventionally known in which image formation units are arranged in tandem that form magenta, cyan, yellow, and black toner images. In such an image formation apparatus, the different colored toner images formed by the different image formation units are transferred to a belt so as to be superimposed on each other, and are then collectively transferred from this belt to paper. On the other hand, when a belt is used to transport paper, different colored toner images are directly transferred from the image formation units to the paper transported by the belt so as to be superimposed on each other. In this image formation apparatus, when the belt runs at an uncertain speed, the images from the different image formation units cannot be superimposed accurately on each other on the belt. This makes color dislocation occur in the image thus formed on the paper.
Therefore, Japanese Patent Application Laid-Open No. H11-65222 proposes an image formation apparatus in which a belt transports paper and the speed of the circumference of a rotating photoconductive drum is so set as to be slower than the rotation speed of the belt. This makes the photoconductive drum that is in close contact with the belt rotate as if it were driven by the belt. As a result, torque of each photoconductive drum is applied to the belt, making it possible to reduce variations in speed of the belt and of each photoconductive drum.
When the image formation apparatus is activated, for example, color dislocation correction and belt speed correction are performed by the use of a toner patch. The color dislocation correction is performed as follows: different colored toner patches formed by the different image formation units and then transferred to the belt are detected by a sensor; then a time difference between times at which the different toner patches are detected is measured; and then control is performed so as to eliminate the time difference thus measured. On the other hand, the belt speed correction is performed according to, for example, the elapsed time between the start of formation of different colored toner patches in the different image formation units and the detection by the sensor of the toner patches thus formed, and to the set rotation speed of the belt. When there is slack in the belt, such a slackened state varies during use. This disturbs the corrected state, leading to occurrence of color dislocation. Therefore, it is necessary to always keep the rotating belt tight between the image formation units.
However, in the image formation apparatus proposed in Japanese Patent Application Laid-Open No. H11-65222, when slack occurs in the belt between the photoconductive drums at the time of activation, the belt tends to remain slack while the image formation apparatus is operating, because the photoconductive drum is in close contact with the belt. This requires time to take up slack in the belt.